How can I make elbows and knees on this without distorting the texture?
First of all you have to transform the tris in quads, or you'll not be able to make loopcuts.





Than make loopcuts in the elbow position with Ctrl+R



What is important is to check the Correct UVs option in the command option panel. This will prevent distortion while working with the new edgeloops.



Same thing when you slide them